Safety pacing in multi-site crm devices

ABSTRACT

Safety pacing in multi-site cardiac rhythm management (CRM) devices is provided. According to various method embodiments, a first cardiac signal from a first cardiac region and a second cardiac signal from a second cardiac region are sensed. The first cardiac region is paced to maintain at least a minimum cardiac rate, and the second cardiac region is paced to maintain at least the minimum cardiac rate when a pace in the first cardiac region is inhibited. Other aspects and embodiments are provided herein.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.11/421,660, filed on Jun. 1, 2006, which is a continuation of U.S.patent application Ser. No. 10/839,268, filed on May 5, 2004, now issuedas U.S. Pat. No. 7,058,449, which is a continuation of U.S. patentapplication Ser. No. 10/291,459, filed on Nov. 8, 2002, now issued asU.S. Pat. No. 6,963,774, which is a continuation of U.S. patentapplication Ser. No. 09/748,721, filed on Dec. 26, 2000, now issued asU.S. Pat. No. 6,480,740, the specifications of which are incorporated byreference herein.

TECHNICAL FIELD

The invention relates generally to cardiac rhythm management systems,and particularly, but not by way of limitation, to a system providing,among other things, reversionary behavior in multi-region pacingtherapy.

BACKGROUND

When functioning properly, the human heart maintains its own intrinsicrhythm, and is capable of pumping adequate blood throughout the body'scirculatory system. However, some people have irregular cardiac rhythms,referred to as cardiac arrhythmias. Such arrhythmias result indiminished blood circulation. One mode of treating cardiac arrhythmiasincludes the use of a cardiac rhythm management system. Such systems areoften implanted in the patient and deliver therapy to the heart.

Cardiac rhythm management systems include, among other things,pacemakers, also referred to as pacers. Pacers deliver timed sequencesof low energy electrical stimuli, called pace pulses, to the heart, suchas via an intravascular lead (referred to as a “lead”) having one ormore electrodes disposed in or about the heart. Heart contractions areinitiated in response to such pace pulses (this is referred to as“capturing” the heart). By properly timing the delivery of pace pulses,the heart can be induced to contract in proper rhythm, greatly improvingits efficiency as a pump. Pacers are often used to treat patients withbradyarrhythmias, that is, hearts that beat too slowly, or irregularly.

Cardiac rhythm management systems also include cardioverters ordefibrillators that are capable of delivering higher energy electricalstimuli to the heart. Defibrillators are often used to treat patientswith tachyarrhythmias, that is, hearts that beat too quickly. Suchtoo-fast heart rhythms also cause diminished blood circulation becausethe heart isn't allowed sufficient time to fill with blood beforecontracting to expel the blood. Such pumping by the heart isinefficient. A defibrillator is capable of delivering a high energyelectrical stimulus that is sometimes referred to as a defibrillationcountershock. The countershock interrupts the tachyarrhythmia, allowingthe heart to reestablish a normal rhythm for the efficient pumping ofblood. In addition to pacers, cardiac rhythm management systems alsoinclude, among other things, pacer/defibrillators that combine thefunctions of pacers and defibrillators, drug delivery devices, and anyother implantable or external systems or devices for diagnosing ortreating cardiac arrhythmias.

One problem faced by cardiac rhythm management systems is the treatmentof heart failure. In some forms, heart failure can be treated bybiventricular coordination therapy that provides pacing pulses to bothright and left ventricles, or by biatrial coordination therapy thatprovides pacing pulses to both the right and left atrium, or othermultichamber coordination therapy. Biventricular and biatrialcoordination therapy each rely on multiple leads to carry out thecoordination therapy of multiple chambers of the heart. In the event ofa failure in one or more of the leads (e.g., failure of an electrode),or in the algorithm controlling the coordination therapy, the benefit ofcoordination therapy may be lost.

As will be seen from the above concerns, there exists a need forimproved failure recovery mechanisms in cardiac rhythm managementsystems used in biventricular and/or biatrial coordination therapy. Theabove-mentioned problems with failure recovery and other problems areaddressed by the various embodiments of the invention and will beunderstood by reading and studying the following specification.

SUMMARY

The various embodiments of the present subject matter include methodsfor pacing site redundancy in a cardiac rhythm management system andapparatus capable of carrying out the methods.

The present subject matter includes an apparatus and method where afirst cardiac signal and a second cardiac signal are sensed. Both thefirst and second cardiac signals include indications of cardiac eventsthat can include intrinsic cardiac events or paced cardiac events. Acardiac rate is determined from the cardiac events in one of the firstcardiac signal or the second cardiac signal, and pacing pulses areprovided to the first cardiac region in order to maintain the cardiacrate at least a minimum rate value.

A pace protection interval starts when a cardiac event is detected inthe first cardiac signal, where the pace protection interval functionsto inhibit delivery of pacing pulses to the first cardiac region. In oneembodiment, the pace protection interval starts in the first cardiacsignal after the intrinsic cardiac event in the first cardiac signal.Alternatively, the pace protection interval starts in the first cardiacsignal after the paced cardiac event in the first cardiac signal.

A cardiac cycle escape time interval is then started after either theintrinsic cardiac event is sensed in the second cardiac signal or thepaced cardiac event is identified in the first or second cardiac signal.The second cardiac signal is analyzed for an intrinsic cardiac eventduring the cardiac cycle escape time interval. When the intrinsiccardiac event is not detected in the second cardiac signal during thecardiac cycle escape time interval, the pacing pulse is provided to thesecond cardiac region at a safety interval timed from the inhibitedpacing pulse to the first cardiac region. In one embodiment, the pacingpulse is provided to the second cardiac site at the end of the cardiaccycle escape time interval when the subsequent intrinsic cardiac eventis not detected in the second cardiac signal during the cardiac cycleescape time interval. In one embodiment, the pacing pulse is provided tothe second cardiac region to maintain the cardiac rate at least theminimum rate value when the pace protection interval inhibits the pacingpulse to the first cardiac region.

In one embodiment, the pacing pulse is provided to the second cardiacregion after the pace protection interval. Alternatively, the pacingpulse is provided to the second cardiac region during, or at the end of,the pace protection interval. In one embodiment, where the pacing pulseis delivered relative the pace protection interval is a function of thesafety interval, where the safety interval timed from the inhibitedpacing pulse is set in the range of zero (0.0) milliseconds to 300milliseconds.

This summary is not intended to be exclusive or exhaustive of allembodiments provided by the present application, and further details arefound in the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing illustrating generally one embodiment ofportions of a cardiac rhythm management system and an environment inwhich it is used;

FIG. 2 is a flowchart showing one embodiment of the present subjectmatter;

FIG. 3A is an illustration of a first cardiac signal and a secondcardiac signal according to the present subject matter;

FIG. 3B is an illustration of a first cardiac signal and a secondcardiac signal according to the present subject matter;

FIG. 3C is an illustration of a first cardiac signal and a secondcardiac signal according to the present subject matter;

FIG. 4 is a flowchart showing one embodiment of the present subjectmatter;

FIG. 5A is an illustration of a first cardiac signal and a secondcardiac signal according to the present subject matter;

FIG. 5B is an illustration of a first cardiac signal and a secondcardiac signal according to the present subject matter;

FIG. 6 is a flowchart showing one embodiment of the present subjectmatter;

FIG. 7 is a schematic drawing illustrating one embodiment of a cardiacrhythm device coupled by leads to a heart;

FIG. 8 is a schematic diagram illustrating generally one embodiment ofportions of a cardiac rhythm management device coupled to a heart; and

FIG. 9 is a flowchart showing one embodiment of the present subjectmatter.

DESCRIPTION OF THE EMBODIMENTS

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that other embodiments may be utilized and thatstructural, logical and electrical changes may be made without departingfrom the spirit and scope of the invention. The following detaileddescription is, therefore, not to be taken in a limiting sense, and thescope of the invention is defined by the appended claims and theirequivalents.

The various embodiments will generally be discussed in the context ofbiventricular pacing therapies, having leads coupled to both the rightand left ventricles. However, the methods described herein can beadapted to biatrial pacing therapies, having leads coupled to both theright and left atrium, as well as other multichamber pacing therapies,e.g., one atrium/two ventricles, two atriums/two ventricles, etc.Furthermore, the methods described herein can be adapted to unichambertherapies, having multiple lead sites within a single chamber.

The presence of multiple lead sites permits useful reversionarybehavior. For example, consider a system in which one lead is implantedin the right ventricle and another lead is implanted adjacent the leftventricle. Suppose that the system was designed to provide pacing pulsesonly to the left ventricle though the lead implanted adjacent the leftventricle. There are a number of situations in which the implantablesignal generator may not deliver therapy through the left ventricularsite. These situations will be specifically outlined below. For now,however, should this situation arise in a pacemaker dependent patient,the patient may be asystolic for as long as the device inhibits thepacing therapy to the left ventricle.

A solution to this problem is to use the inherent redundancy provided bythe right ventricular lead. The present subject matter allows for apacing pulse to be delivered to an alternative site when a pacing pulseis inhibited from being delivered to its primary, or typical, site. Thisallows at least a minimum pacing rate to be maintained even when thepacing pulses are inhibited from being delivered to their primary site.

There are many reasons why a system in which a cardiac signal from botha first cardiac region and a second cardiac region are sensed andanalyzed, but only delivers pacing pulses to the first cardiac region,may be unable to deliver the programmed pacing therapy. These can begrouped into two general classes. The first class includes therapyinhibition due to failures and the second class includes therapyinhibition due to normal algorithm behavior. Examples in the first class(failures) include oversensing, lead failure, lead-tissue interfacefailure and memory failure. These, and other, examples of failures, andhow the failures are addressed, are found in U.S. patent applicationSer. No. 09/650,568 entitled “Site Reversion in Cardiac RhythmManagement” filed on Aug. 30, 2000, now issued as U.S. Pat. No.6,493,586, that is hereby incorporated by reference in its entirety.Examples of the second class include algorithms that are designed toinhibit pacing during specified time intervals following a sensed orpaced cardiac event at a primary pacing site. Since pacing pulses arenot delivered in this situation, the patient's cardiac rate may fallbelow a predetermined minimum value. This is an undesirable situationwhich the present subject matter addresses.

FIG. 1 is a schematic drawing illustrating, by way of example, but notby way of limitation, one embodiment of portions of a cardiac rhythmmanagement system 100 and an environment in which it is used. In FIG. 1,system 100 includes an implantable signal generator 105 that is coupledby a first cardiac lead 110 and a second cardiac lead 115, or one ormore additional leads, to a heart 120 of a patient 125. Implantablesignal generator 105 can take the form of an implantable pacemaker or animplantable cardioverter/defibrillator that includes pacing capability.The implantable signal generator 105 is adapted to perform the methodsas described herein. System 100 also includes an external programmer 140that provides for wireless communication with pacemaker 105 using atelemetry device 145. The first cardiac lead 110 and the second cardiaclead 115 each include a proximal end and a distal end, where the distalend of the leads 110 and 115 are implanted in, or on, the heart 120 at afirst cardiac region and a second cardiac region, respectively. Eachlead includes one or more electrodes, as will be described below, thatallow for combinations of either unipolar and/or bipolar sensing anddelivery of energy to the heart 120 for pacing, cardioversion and/ordefibrillation.

FIG. 2 is a flow chart illustrating one embodiment of a method 200according to the present subject matter. At 210, a first cardiac signalis sensed from a first cardiac region and a second cardiac signal issensed from a second cardiac region. In one embodiment, the firstcardiac signal is sensed from a left ventricular region through the useof one or more electrodes on a cardiac lead. The cardiac lead ispositioned adjacent the left ventricle, where the lead is eitherimplanted transvenously (e.g., positioned through the coronaryvasculature) or implanted epicardially on the surface of the heart so asto position the one or more electrodes adjacent the left ventricle. Inone embodiment, the one or more electrodes sense either unipolar orbipolar cardiac signals from the left ventricular region, as will bedescribed in greater detail below.

Additionally, the second cardiac signal is sensed from a rightventricular region through the use of one or more electrodes on thecardiac lead. The cardiac lead is positioned with its distal end in theright ventricle, where in one embodiment the distal end of the lead ispositioned in the apex of the right ventricle. Other locations withinthe right ventricle are, however, possible for implanting the distal endof the cardiac lead. In one embodiment, the one or more electrodes senseeither unipolar or bipolar cardiac signals from the right ventricularregion, as will be described in greater detail below. In an alternativeembodiment, the first cardiac signal is sensed from a right ventricularchamber and the second cardiac signal is sensed from a left ventricularchamber.

In addition to ventricular locations, other cardiac locations exist fromwhich the first and second cardiac signals can be sensed. For example,the first and second cardiac signals are sensed from left and rightatrial chambers, respectively, or from right and left atrial chamber,respectively. Alternatively, the first and second cardiac signals aresensed from two different regions within the same cardiac chamber (e.g.,either right atrium or ventricle, or adjacent left atrium or ventricle).

The first cardiac signal and the second cardiac signal each includeindications of cardiac events. The type of cardiac event sensed in eachof the cardiac signals depends upon the type of electrodes used and thelocation of the implanted electrodes. Examples of sensed cardiac eventsinclude sensed P-waves from atrial depolarization and R-waves and/orQRS-complexes from ventricular depolarizations. From this information, acardiac rate is determined at 220 from the cardiac events in one ofeither the first cardiac signal or the second cardiac signal. In oneembodiment, the cardiac rate is determined from intrinsic cardiaccontractions detected in either the first or second cardiac signal.Alternatively, the cardiac rate is determined from paced cardiaccontractions detected in the first or second cardiac signal.

Based on the cardiac rate, at 230 pacing pulses are provided to thefirst cardiac region to maintain the cardiac rate at least a minimumrate value. In one embodiment, the minimum rate value is a programmablevalue and represents the minimum cardiac rate which the implantablesignal generator will maintain for the heart. When the heart's ownintrinsic rhythm is sufficient to maintain the cardiac rate at theminimum rate value, providing the pacing pulses to the first cardiacregion may not be required. Alternatively, when the intrinsic rhythm isinsufficient to maintain the cardiac rate at the minimum rate value,pacing pulses are delivered, as necessary, to maintain at least thisminimum cardiac rate.

At 240, a pace protection interval is started for the first cardiacsignal after a cardiac event occurs in the first cardiac signal. Thecardiac events in the first cardiac signal are the result of either apacing pulse delivered to the first cardiac region or an intrinsiccontraction sensed from the first cardiac region or possibly a far-fieldsense of an intrinsic contraction of the second cardiac region ornon-cardiac signals. The pace protection interval is a time intervalduring which pacing pulses to the first cardiac region are inhibited. Inone embodiment, the pace protection interval is a programmable timeinterval that is set in the range of 200 milliseconds to 500milliseconds, where 400 milliseconds is a possible value for the paceprotection interval.

During the pace protection interval pacing pulses to the first cardiacregion are inhibited. In one embodiment, pacing pulses to the firstcardiac region are inhibited during a vulnerable time of the cardiaccycle in the first cardiac region. Examples of these vulnerable timesinclude the occurrence of the T-wave in the first cardiac region, wherethe T-wave occurs during the repolarization of the cardiac tissue in thefirst region. Pacing pulses delivered to the cardiac tissue during theT-wave can cause an arrhythmia, such as a fibrillation, to occur. Thus,in an effort to avoid inducing an arrhythmia, pacing pulses areinhibited during the pace protection interval.

While pacing pulses to the first cardiac region are inhibited, pacingpulses are not inhibited from being provided to the second cardiacregion. At 250, the pacing pulse is provided to the second cardiacregion at a safety interval timed from the inhibited pacing pulse to thefirst cardiac region. In one embodiment, the pacing pulse is provided tothe second cardiac region to maintain the cardiac rate at least theminimum rate value when the pace protection interval inhibits the pacingpulse to the first cardiac region. In one embodiment, the timing of thepacing pulses delivered to the second cardiac region is based on acardiac cycle escape time interval that is started once a cardiac eventis detected in the second cardiac signal. In this example, the pacingpulse is to be provided to the second cardiac region when the cardiaccycle escape time interval expires.

In another embodiment, the timing of the pacing pulses delivered to thesecond cardiac region is based on a safety interval timed from theinhibited pacing pulse to the first cardiac region.

One possible operating range of the safety interval is the range of zero(0.0) milliseconds to 300 milliseconds which corresponds to the typicalrange of intrinsic atrio-ventricular conduction delays. Thus, when thesafety interval has a value of 0 milliseconds, the pacing pulse isprovided to the second cardiac region at the time when the pacing pulsewas to be delivered to the first cardiac region. In this case, thepacing pulse is provided to the second cardiac region during the paceprotection interval. In an additional embodiment, the safety interval isprogrammed such that the pacing pulse is provided to the second cardiacregion at the end of the pacing protection interval or after the paceprotection interval. Other times and locations relating the inhibitedpacing pulse to the first cardiac region for delivering the pacing pulseto the second cardiac region are also possible.

This embodiment has the advantage of timing a pacing pulse to the timeof the inhibited pace when that pace is scheduled to occur before acardiac cycle escape time interval has expired (e.g., in atrial trackingmode after the atrio-ventricular delay). In addition, when the safetyinterval is programmed to zero (0) milliseconds, the pacing pulse isprovided to the second cardiac region at the end of the cardiac cycleescape time interval.

The cardiac cycle escape time interval is an interval, measured inmilliseconds, between a cardiac event (e.g., a sensed intrinsic event ora paced event in the second cardiac region) and a subsequently deliveredpacing pulse. During the cardiac cycle escape time interval, when anintrinsic cardiac event is sensed in the second cardiac signal, thecardiac cycle escape time interval would terminate and a new cardiaccycle escape time would begin. However, when an intrinsic cardiac eventis not sensed in the second cardiac signal during the time interval, apacing pulse is delivered to the primary pacing location at the end ofthe cardiac cycle escape time interval. In certain situations, as willbe elaborated below, pacing pulses to be delivered at the end of thecardiac cycle escape time interval are inhibited from being delivered tothe primary pacing location. To prevent the cardiac rate from fallingbelow the minimum cardiac rate, a “safety” pace (or a back-up pace) isdelivered to a secondary pacing location. In the present embodiment, thesecondary pacing location is the second cardiac region, while theprimary pacing location is the first cardiac region.

FIG. 3A provides an example of the present subject matter. In FIG. 3A,there is shown a first cardiac signal 300 and a second cardiac signal304. In one embodiment, the first cardiac signal 300 is sensed from afirst cardiac region, and the second cardiac signal 304 is sensed from asecond cardiac region. As previously discussed, the first and secondcardiac regions include combinations of the left ventricle, the rightventricle, the left atrium, and the right atrium. The cardiac regionscan also include two regions within, or adjacent to, any of theaforementioned cardiac chambers. For the example in FIG. 3A, the firstcardiac signal 300 is sensed from a location adjacent the left ventricleand the second cardiac signal 304 is sensed from the right ventricle.

The example shown in FIG. 3A illustrates an instance when a pacing pulseis delivered to a right ventricular location during a left ventricularonly pacing mode. In other words, in this bi-ventricular system, cardiacsignals are sensed from both the right and left ventricle, but theprimary pacing location is the left ventricle. An intrinsic cardiaccontraction is detected in both the first and second cardiac signals,300 and 304, where the cardiac contraction is shown at 308 in the firstcardiac signal 300 and at 310 in the second cardiac signal 304. Theintrinsic cardiac contractions 308 and 310 trigger the start of a paceprotection interval 314 in the first cardiac signal 300, and a cardiaccycle escape time interval 318 in the second cardiac signal 304,respectively.

Cardiac signals 300 and 304 are then analyzed for the presence ofintrinsic cardiac events that occur during the pace protection interval314 and the cardiac cycle escape time interval 318. In one embodiment,when an intrinsic cardiac event is detected in the right ventricle(i.e., the second cardiac signal 304), the cardiac cycle escape timeinterval 318 is restarted. If, however, an intrinsic cardiac signal isnot detected in the first or second cardiac signals during the cardiaccycle escape time interval 318, a pacing pulse 330 should be deliveredto the first cardiac region (the left ventricular region in thisexample) at the end of the time interval 318. The pacing pulse, however,would be delivered during the pace protection interval 314, and thepacing pulse is therefore inhibited.

With the pacing pulse 330 to the first cardiac region inhibited, thereis the possibility that the cardiac rate will fall below the minimumrate value. To prevent the cardiac rate from falling below the minimumcardiac rate, a pacing pulse 340 is delivered not to the first cardiacregion, but to the second cardiac region (the right ventricle region inthis example). After the pacing pulse 340, the pacing pulses deliveredto the first cardiac region is resumed in a next cardiac cycle 350.

In one embodiment, the delay between the cardiac contractions in theright and left ventricles could be due to at least two reasons. First,the contraction in the left ventricle might lag the contraction in theright ventricle, as shown in FIG. 3A, due to a left bundle branch blockresulting in a delayed depolarization on the left ventricle. Second, thecontraction in the left ventricle might lag the contraction in the rightventricle due to a premature ventricular contraction (PVC) originatingin the right ventricle.

FIG. 3B provides an additional example of the present subject matter,where the timing of the pacing pulse delivered to the second cardiacregion is based on the safety interval from the time when the inhibitedpace to the first cardiac region was to have occurred. In FIG. 3B, thereis shown the first cardiac signal 300 and the second cardiac signal 304,as previously described. In addition, an atrial cardiac signal 354 issensed from an atrial location, where the atrial signal 354 includesindications of atrial contractions 356. The example shown in FIG. 3Billustrates a dual chamber implantable pulse generator system, wherecardiac signals are sensed from an atrial location and from the rightand left ventricles. In this example, the atrial contraction 356 is usedto time an atrioventricular interval (AVI) 360. The AVI 360 is theinterval between the atrial event 356 and the paced ventricular event.In this example the paced ventricular event should occur at 330, but thepacing pulse 330 is inhibited due to the pace protection interval 314.Instead, pacing pulse 340 is delivered to the second cardiac region. Inone embodiment, a safety interval 364 is used to delay the delivery ofthe pacing pulse 340. This situation allows the pacing pulse 340 to betimed to the inhibited pacing pulse 330 when the inhibited pacing pulse330 was scheduled to occur before a cardiac cycle escape time intervalhas expired. After the pacing pulse 340, the pacing pulses delivered tothe first cardiac region is resumed in a next cardiac cycle 350.

FIG. 3C provides an additional example of the present subject matter,where the example of FIG. 3C shows an asynchronous pacing mode whereonly left ventricular events are sensed and right ventricular sensedevents are not used to time any pacing events. In FIG. 3C, there isshown the first cardiac signal 300 and the second cardiac signal 304,detected as previously described. In addition, a cardiac cycle escapetime interval 380 is shown, where the cardiac cycle escape time interval380 is started after a right or left ventricular paced event. During thecardiac cycle escape time interval 380, a cardiac contraction is shownat 308 in the first cardiac signal 300 and at 310 in the second cardiacsignal 304. In this example, the cardiac contraction 308 detected in thefirst cardiac signal 300 triggers the start of a pace protectioninterval 314. However, the cardiac contraction 310 detected in thesecond cardiac signal 304 is ignored (i.e., the cardiac cycle escapetime interval 380 is not reset) due the system operating in anasynchronous pacing mode. One example of this type of mode is a VOOmode, where cardiac contractions in the left ventricular signal aresensed to start the pace protection interval 314, but cardiac signalsare not sensed to determine the cardiac cycle time.

In the present example the paced ventricular event should occur at 330when the cardiac cycle escape time interval 380 expires. However, thepacing pulse 330 is inhibited due to the pace protection interval 314.Instead, pacing pulse 340 is delivered to the second cardiac region. Inone embodiment, a safety interval 364 is used to delay the delivery ofthe pacing pulse 340. After the pacing pulse 340, the pacing pulsesdelivered to the first cardiac region is resumed in a next cardiac cycle350.

FIG. 4 is a flow chart illustrating an embodiment of a method 400according to the present subject matter. At 410, a first cardiac signalis sensed from a first cardiac region and a second cardiac signal issensed from a second cardiac region, as discussed for FIG. 2. At 420,intrinsic cardiac events are detected in the sensed second cardiacsignal. As previously discussed, intrinsic cardiac events include, butare not limited to, P-waves as sensed from an atrial location, orR-waves and/or QRS-complexes as sensed from ventricular locations. Fromthis information, a cardiac rate is determined at 430 from the cardiacevents in the second cardiac signal. Pacing pulses are provided, ifnecessary, to the first cardiac region at 440 to maintain the cardiacrate at least a minimum rate value. However, when the heart's ownintrinsic rhythm is sufficient to maintain the cardiac rate at theminimum rate value, providing the pacing pulses to the first cardiacregion may not be required.

At 450, an intrinsic cardiac contraction is detected in both the firstcardiac signal and the second cardiac signal. At 460, the paceprotection interval in the first cardiac signal is started after theintrinsic cardiac event is detected in the first cardiac signal. Inaddition, a cardiac cycle escape time interval is started after theintrinsic cardiac event is detected in the second cardiac signal. Aspreviously mentioned, the pace protection interval is a programmabletime interval that is set in the range of 200 milliseconds to 500milliseconds, where 400 milliseconds is a possible value for the paceprotection interval. In one embodiment, the cardiac cycle escape timeinterval is also a programmable time interval in the range of 340 to2000 milliseconds.

At 470, the second cardiac signal is analyzed to detect a subsequentintrinsic cardiac event in the second cardiac signal during the cardiaccycle escape time interval. When an intrinsic cardiac event is detectedin the second cardiac signal, the cardiac cycle escape time interval isstarted again by returning to 460. When a subsequent intrinsic cardiacevent is not detected in the second cardiac signal during the cardiaccycle escape time interval, however, a pacing pulse is delivered to thesecond cardiac site at the end of the cardiac cycle escape timeinterval, as shown at 480.

FIG. 5A provides an additional example of the present subject matter. InFIG. 5A, there is shown a first cardiac signal 500 and a second cardiacsignal 504. In one embodiment, the first cardiac signal 500 is sensedfrom a first cardiac region, and the second cardiac signal 504 is sensedfrom a second cardiac region. As previously discussed, the first andsecond cardiac regions include combinations of the left ventricle, theright ventricle, the left atrium, and the right atrium. The cardiacregions can also include two regions within, or adjacent to, any of theaforementioned cardiac chambers. For the example in FIG. 5A, the firstcardiac signal 500 is sensed from a location adjacent the left ventricleand the second cardiac signal 504 is sensed from the right ventricle.

The example shown in FIG. 5A illustrates an instance when a pacing pulseis delivered to a right ventricular location during a left ventricularonly pacing mode. In other words, in this bi-ventricular system, cardiacsignals are sensed from both the right and left ventricle, but theprimary pacing location is the left ventricle. As the cardiac signalsare being sensed, a paced cardiac event is identified in each of thefirst and second cardiac signals, 500 and 504, where the paced cardiacevent in the first cardiac signal 500 is shown at 508 and the pacedcardiac event in the second cardiac signal 504 is shown at 510. Thesensed paced cardiac events 508 and 510 trigger the start of a paceprotection interval 514 in the first cardiac signal 500, and a cardiaccycle escape time interval 518 in the second cardiac signal 504.

Cardiac signals 500 and 504 are then analyzed for the presence ofintrinsic cardiac events that occur during the pace protection interval514 and the cardiac cycle escape time interval 518. In one embodiment,when an intrinsic cardiac event is detected in the right ventricle(i.e., the second cardiac signal 504), the cardiac cycle escape timeinterval 518 is restarted. If, however, an intrinsic cardiac signal isnot detected in the first or second cardiac signals during the cardiaccycle escape time interval 518, a pacing pulse 530 should be deliveredto the first cardiac region (the left ventricular region in thisexample) at the end of the time interval 518. The pacing pulse, however,would be delivered during the pace protection interval 514, and thepacing pulse is therefore inhibited.

With the pacing pulse 530 to the first cardiac region inhibited, thereis the possibility that the cardiac rate will fall below the minimumrate value. To prevent the cardiac rate from falling below the minimumcardiac rate, a pacing pulse 540 is delivered not to the first cardiacregion, but to the second cardiac region (the right ventricle region inthis example). After the pacing pulse 540, the pacing pulses deliveredto the first cardiac region are resumed at 550 in a next cardiac cycle.

In one embodiment, FIG. 5A illustrates an example where a pacing pulseis invoked when a biventricular cardiac management system istransitioning from a biventricular pacing mode with a positive leftventricular offset (i.e., the pacing pulse to the left ventricle followsthe pacing pulse delivered to the right ventricle by a set timeinterval) to a left ventricle only pacing mode. The paced leftventricular and right ventricular events trigger the pace protectioninterval and the cardiac cycle escape time interval, as previouslydiscussed. As no intrinsic cardiac events were sensed during the paceprotection interval and the cardiac cycle escape time interval, a pacingpulse 540 was delivered to the second cardiac region (the rightventricular region in this instance). The transition to the leftventricular only pacing mode then takes place with the pacing pulsedelivered at 550 to start the next cardiac cycle.

FIG. 5B provides an additional example of the present subject matter,where the timing of the pacing pulse delivered to the second cardiacregion is based on the safety interval from the time when the inhibitedpace to the first cardiac region was to have occurred. FIG. 5Billustrates an example where a pacing pulse is invoked when abiventricular cardiac management system is transitioning from afixed-rate pacing mode (i.e., where pacing rate is independent of theheart's intrinsic rate) to an atrial tracking mode (where pacing followsthe intrinsic atrial rate). In FIG. 5B, there is shown the first cardiacsignal 500 and the second cardiac signal 504, as previously described.In addition, an atrial cardiac signal 556 is sensed from an atriallocation, where the atrial signal 556 includes indications of a pacedatrial event 558. The example shown in FIG. 5B illustrates a dualchamber implantable pulse generator system, where cardiac signals aresensed from an atrial location and from the right and left ventricles.In this example, the paced atrial event 558 is used to time anatrioventricular interval (AVI) 560. The AVI 560 is the interval betweenthe paced atrial event 558 and the paced ventricular event. In thisexample the paced ventricular event should occur at 530, but the pacingpulse 530 is inhibited due to the pace protection interval 514. Instead,pacing pulse 540 is delivered to the second cardiac region. In oneembodiment, a safety interval 564 is used to delay the delivery of thepacing pulse 540. This situation allows the pacing pulse 540 to be timedto the inhibited pacing pulse 530 when the inhibited pacing pulse 530was scheduled to occur before a cardiac cycle escape time interval hasexpired. After the pacing pulse 540, the pacing pulses delivered to thefirst cardiac region is resumed in a next cardiac cycle 550.

FIG. 6 is a flow chart illustrating an embodiment of a method 600according to the present subject matter. At 610, a first cardiac signalis sensed from a first cardiac region and a second cardiac signal issensed from a second cardiac region, as discussed for FIG. 2. At 620,intrinsic cardiac events are detected in the sensed second cardiacsignal. As previously discussed, intrinsic cardiac events include, butare not limited to, P-waves as sensed from an atrial location, orR-waves and/or QRS-complexes as sensed from ventricular locations. Fromthis information, a cardiac rate is determined at 630 from the cardiacevents in the second cardiac signal. Pacing pulses are provided, ifnecessary, to the first cardiac region at 640 to maintain the cardiacrate at least a minimum rate value. However, when the heart's ownintrinsic rhythm is sufficient to maintain the cardiac rate at theminimum rate value, providing the pacing pulses to the first cardiacregion may not be required.

At 650, a paced cardiac event is identified in both the first cardiacsignal and the second cardiac signal. At 660, the pace protectioninterval in the first cardiac signal is started after the paced cardiacevent is identified in the first cardiac signal. In addition, a cardiaccycle escape time interval is started after the paced cardiac event isidentified in the second cardiac signal. At 670, the second cardiacsignal is analyzed to detect a subsequent intrinsic cardiac event in thesecond cardiac signal during the cardiac cycle escape time interval.When an intrinsic cardiac event is detected in the second cardiacsignal, the cardiac cycle escape time interval is started again byreturning to 660. When a subsequent intrinsic cardiac event is notdetected in the second cardiac signal during the cardiac cycle escapetime interval, however, a pacing pulse is delivered to the secondcardiac site at the end of the cardiac cycle escape time interval, asshown at 680.

In addition to the embodiments described above, the location of thefirst cardiac region and the second cardiac region can be reversed.Thus, the first cardiac signal is sensed from the right ventricle andthe second cardiac signal is sensed from the left ventricle. Signalprocessing and analysis occur as described, but with the first cardiacregion being the right ventricle and the second cardiac region being theleft ventricle. In this embodiment, safety paces would be delivered tothe left ventricle instead of the right ventricle.

A still further example of a failure indication can be based on multiplesite cross-checking Excitatory heart tissue naturally propagateselectrical activation from one region to another (conduction) as part ofits function to generate coordinated contraction within a heart chamberand between left and right chambers of the heart. With a multisitedesign, multiple leads and/or electrodes are placed within a singleheart chamber and/or in both left and right chambers. Due to naturalelectrical propagation in heart tissue, activation sensed or initiatedby pacing at one site will be followed after a predictable conductiondelay by sensed activation at another site. Thus, verification of pacecapture or sensed activation at one site can be cross-checked by sensingconducted events at another site. For example, if paced capture occursat a primary site, activation will be sensed at a second site after aconduction delay dependent largely on the distance separating the twosites. If activation is not sensed at the second site within an expectedconduction delay window, a failed capture condition is detected at thepacing site and site reversion could occur for a backup pace or forpacing on subsequent cardiac cycles. As another example, if activationis detected by sensing at a primary site, a paired activation will bedetected at a second sensing site either just earlier or just later thanat the primary site due to conduction. If activation is not detected atthe second sensing site within the expected conduction delay windowbefore and/or after detection at the primary site, the detection at theprimary site may be due to a failure condition (e.g., sensed noise,over-sensing) and sensing site reversion can occur.

FIG. 7 is a schematic drawing illustrating, by way of example, but notby way of limitation, one embodiment of an apparatus 700 that includes asignal generator 105 coupled by leads 110 and 115 to a heart 715. In oneembodiment, lead 110 has at least a first sensing/pacing electrodeadapted for connection to the signal generator 105, and lead 115 has asecond sensing/pacing electrode also adapted for connection to thesignal generator 105. The heart 715 includes a right atrium 718, a leftatrium 720, a right ventricle 722, a left ventricle 724, and a coronarysinus 726 extending from right atrium 718. In one such embodiment, theapparatus 700 provides biventricular coordination therapy to coordinateright ventricular and left ventricular contractions, such as forcongestive heart failure patients. The system 700 also contains controlcircuitry within housing 730 that receives the cardiac signals andperforms the present subject matter.

In one embodiment, lead 110 is shown with the first sensing/pacingelectrode 734 along with an additional sensing/pacing electrode 736.Lead 110 is shown inserted through coronary sinus 726 and into the greatcardiac vein so that electrodes 734 and 736 are adjacent the leftventricle 724, where the electrodes 734 and 736 are used to senseintrinsic heart signals and provide one or more coordination pacespulses. In one embodiment, electrodes 734 and 736 are ring electrodesthat partially or completely surround lead 110. Alternatively, electrode736 is a ring electrode and electrode 734 is a tip electrode. In anadditional embodiment, lead 110 also includes an additional electrodepositioned proximal electrodes 734 and 736, where the additionalelectrode can be used in combination with electrodes 734 and 736 toprovide bipolar and unipolar pacing and sensing from the leftventricular lead 110.

Lead 115 is shown with the second sensing/pacing electrode 740, alongwith a first defibrillation coil electrode 744 and a seconddefibrillation coil electrode 750. The second sensing/pacing electrode740 and the first defibrillation coil electrode 744 are shown disposedin, around, or near the right ventricle 722, for delivering sensingsignals and/or delivering pacing therapy. The second defibrillation coilelectrode 750 is positioned proximal the first defibrillation coilelectrode 744 so as to position the second defibrillation coil electrode750 at least partially in the right atrium 718. The first and seconddefibrillation coil electrodes 744 and 750 are then used to deliveratrial and/or ventricular cardioversion/defibrillation therapy to heart715. In addition, the housing 730 of the signal generator 105 is used asan optional electrode in sensing cardiac signals and deliveringelectrical energy to the heart in conjunction with any of theaforementioned electrodes. In addition, the apparatus 700 furtherincludes an atrial lead, where the atrial lead includes at least onepace/sense electrode to allow for an atrial cardiac signal to be sensedand for atrial cardiac events to be sensed. The atrial signal is thenused in the present subject matter in conjunction with dual chamberdevices, as described in FIGS. 3B and 5B.

In FIG. 7, device 105 includes components, such as the electroniccontrol circuitry, enclosed in the hermetically-sealed housing 730.Additional electrodes may be located on the housing 730, may be thehousing 730 itself, may be on an insulating header 760, or on otherportions of device 105, for providing unipolar or bipolar pacing/sensingand/or defibrillation energy in conjunction with the electrodes disposedon or around heart 715. Other forms of electrodes include meshes andpatches which may be applied to portions of heart 715 or which may beimplanted in other areas of the body to help “steer” electrical currentsproduced by device 105. The present method and apparatus will work in avariety of configurations and with a variety of electrical contacts or“electrodes.”

FIG. 8 is a schematic diagram illustrating generally, by way of example,but not by way of limitation, one embodiment of control circuitry 800 ofthe signal generator 105. The signal generator 105, as shown in FIG. 8,includes a power source 802, a first cardiac signal sensor 808, a secondcardiac signal sensor 810, a cardiac signal analyzer 814, a pacingoutput circuit 820, a pace protection circuit 824 and a control circuit830. In one embodiment, the control circuit 830 incorporates amicroprocessor for controlling the signal generator 105. In oneembodiment, the function of the pace protection circuit 824 isimplemented in software within control circuit 830. In addition, thepace protection interval can also be implemented in software within thecontrol circuit 830.

The first cardiac signal sensor 808 is coupled to the firstsensing/pacing electrode on the first cardiac lead 110. In oneembodiment, the first cardiac signal sensor 808 uses the firstsensing/pacing electrode, along with the additional electrode on 110 orthe housing 770, to sense the first cardiac signal as previouslydescribed. As discussed, the first cardiac signal is either a unipolarcardiac signal or a bipolar cardiac signal, depending upon theelectrodes used in sensing the signal. The second cardiac signal sensor810 is coupled to the second sensing/pacing electrode on the secondcardiac lead 115. In one embodiment, the second cardiac signal sensor810 uses the second sensing/pacing electrode 740, along with the firstdefibrillation coil electrode 744 or the housing 770, to sense thesecond cardiac signal as previously described. As discussed, the secondcardiac signal is either a unipolar cardiac signal or a bipolar cardiacsignal, depending upon the electrodes used in sensing the signal.

The cardiac signal analyzer 814 receives both the first cardiac signaland the second cardiac signal. The cardiac signal analyzer 814determines a cardiac rate from the cardiac events in one of the firstcardiac signal or the second cardiac signal, as previously discussed.Based on the cardiac rate, the pacing output circuit 820, under thecontrol of the control circuit 830, provides pacing pulses to the firstsensing/pacing electrode and the second sensing/pacing electrode tomaintain the cardiac rate at least the minimum rate value. In oneembodiment, the minimum rate value is a programmable value that isstored in a memory 836 of the control circuitry 800.

In one embodiment, the cardiac signal analyzer 814 detects intrinsic oridentifies paced cardiac event in the first and second cardiac signals.When this occurs, the pace protection circuit 824 starts a paceprotection interval in the first cardiac signal, as previouslydescribed. The pace protection circuit 824 also starts the cardiac cycleescape time interval after the intrinsic or paced cardiac event issensed or identified in the second cardiac signal, as previouslydescribed. The pace protection circuit 824 then inhibits pacing pulsesfrom the pacing output circuit 820 to the first cardiac region duringthe pace protection interval.

The cardiac signal analyzer 814 also senses subsequent intrinsic cardiacevents in the second cardiac signal during the cardiac cycle escape timeinterval. When a subsequent intrinsic cardiac event is not detected inthe second cardiac signal, the pace protection circuit 824 causes thepacing output circuit 820 to provide a pacing pulse to the secondsensing/pacing electrode at the safety interval timed from the inhibitedpacing pulse to the first cardiac region. As previously mentioned, thesafety interval is programmed in the range of zero (0.0) milliseconds to300 milliseconds. The safety interval allows the pacing pulse to beprovided to the second cardiac region either during the pace protectioninterval, which includes at the end of the pace protection interval, orafter the pace protection interval has ended.

As previously described, one reason for providing the pacing pulse tothe second cardiac region is to maintain the cardiac rate at least theminimum rate value. In addition to providing pacing pulses to the secondsensing/pacing electrode, the pace protection circuit 824 causes thepacing output circuit 820 to provide a pacing pulse to the firstsensing/pacing electrode to maintain the cardiac rate at least theminimum rate value when the pace protection circuit 824 does not inhibitthe pacing pulse to the first sensing/pacing electrode.

FIG. 9 is a flow chart illustrating an additional embodiment of a method900 according to the present subject matter. The method 900 starts at910, where the first cardiac signal is monitored from the first cardiacregion and the second cardiac signal is monitored from the secondcardiac region. During the monitoring at 910, when a cardiac event isdetected in the first cardiac region, but not in the second cardiacregion the method 900 proceeds to 920 where the pacing protectioninterval is started for the first cardiac signal. Also at 920, thesecond cardiac signal continues to be monitored from the second cardiacregion.

When a cardiac event is detected in the second cardiac signal from thesecond cardiac region, the method 900 proceeds to 930 where the cardiaccycle escape time interval is reset based on the sensed cardiac eventfrom the second cardiac region. At 920, when no additional cardiacevents are detected in the second cardiac signal by the end of thecardiac cycle escape time interval, the method 900 proceeds to 940. At940, an inquiry as to whether the pacing protection interval in thefirst cardiac signal has expired is made. When the pacing protectioninterval in the first cardiac signal has expired, the method 900proceeds to 950. At 950, the pacing pulse is provided to the firstcardiac region to maintain the cardiac rate at least the minimum ratevalue. The method 900 then proceeds to 930 where the cardiac cycleescape time interval is reset based on the pacing pulse provided to thefirst cardiac region. At 940, when the pacing protection interval in thefirst cardiac signal has not expired, the method 900 proceeds to 960. At960, the pacing pulse is provided to the second cardiac region tomaintain the cardiac rate at least the minimum rate value. The method900 then proceeds to 930 where the cardiac cycle escape time interval isreset based on the pacing pulse provided to the second cardiac region.

Referring again to 910, when no cardiac events are detected in the firstor second cardiac signals by the end of the cardiac cycle escape timeinterval, the method 900 proceeds to 950. At 950, the pacing pulse isprovided to the first cardiac region to maintain the cardiac rate atleast the minimum rate value. The method 900 then proceeds to 930 wherethe cardiac cycle escape time interval is reset based on the pacingpulse provided to the first cardiac region.

CONCLUSION

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. Many adaptations ofthe invention will be apparent to those of ordinary skill in the art.For example, use of markers or other display mechanisms to indicatereversion or the number of reversions may be used to assist thephysician during interrogation of the cardiac rhythm management device.Different lead types, numbers of leads, and utilized heart chambers canbe varied from the examples depicted herein. Accordingly, thisapplication is intended to cover any adaptations or variations of theinvention. It is manifestly intended that this invention be limited onlyby the following claims and equivalents thereof.

1. A method for using at least two electrodes implanted in or proximateto a heart chamber, wherein the at least two electrodes include a firstelectrode and a second electrode, the method comprising: using the firstelectrode operationally positioned proximate to a first site of theheart chamber to pace the first site or to sense a cardiac activation atthe first site, wherein the second electrode is not used with the firstelectrode to pace the first site; and using the second electrodeoperationally positioned proximate to a second site of the heart chamberto provide pacing redundancy for the heart chamber by pacing the secondsite if a failure or an algorithm inhibits a pace to the first site orto provide sensing redundancy for the heart chamber, wherein the secondelectrode is used instead of the first electrode to pace the heartchamber if the second electrode provides pacing redundancy for the heartchamber.
 2. The method of claim 1, wherein: using the first electrodeincludes pacing the first site of the heart chamber using the firstelectrode; and using the second electrode includes sensing cardiacactivation at the second site of the heart chamber using the secondelectrode and determining if cardiac activation at the second siteoccurs within an expected conduction delay window from a time when thefirst site is paced.
 3. The method of claim 1, wherein: using the firstelectrode includes sensing the cardiac activation at the first site ofthe heart chamber using the first electrode; and using the secondelectrode includes sensing cardiac activation at the second site of theheart chamber using the second electrode and determining if the cardiacactivation at the second site occurs within an expected conduction delaywindow from the cardiac activation at the first site.
 4. A method forusing multiple electrodes implanted in or proximate to the leftventricle, the method comprising: using a first electrode operationallypositioned proximate to a first site of the heart chamber to pace thefirst site or to sense a cardiac activation at the first site, whereinthe second electrode is not used with the first electrode to pace thefirst site or to sense the cardiac activation at the first site; andusing at least one alternative electrode to the first electrode if afailure or an algorithm inhibits a pace to the first site of the leftventricle or to provide sensing redundancy for the left ventricle,wherein the at least one alternative electrode is operationallypositioned to the left ventricle and is used to provide pacingredundancy for the left ventricle by pacing the left ventricle at analternative site to the first site if a failure or an algorithm inhibitsa pace to the first site or to provide sensing redundancy for the heartchamber, wherein the at least one alternative electrode is used insteadof the first electrode to pace the heart if the at least one alternativeelectrode provides pacing redundancy for the left ventricle.
 5. Themethod of claim 4, wherein: using the first electrode includes pacingthe first site of the left ventricle using the first electrode; andusing the at least one alternative electrode includes sensing cardiacactivation at the alternative site of the left ventricle using the atleast one alternative electrode and determining if cardiac activation atthe alternative site occurs within an expected conduction delay windowfrom a time when the first site is paced.
 6. The method of claim 4,wherein: using the first electrode includes sensing the cardiacactivation at the first site of the left ventricle using the firstelectrode; and using the at least one alternative electrode includessensing cardiac activation at the alternate site of the left ventricleusing the at least one alternative electrode and determining if thecardiac activation at the alternate site occurs within an expectedconduction delay window from the cardiac activation at the first site.7. A method for using multiple electrodes implanted in or proximate tothe left ventricle, the method comprising: using a first electrodeoperationally positioned proximate to a first site of the left ventricleto pace the first site or to sense a cardiac activation at the firstsite; and using at least one additional electrode to the first electrodeif a failure or an algorithm inhibits a pace to the first site of theleft ventricle or to provide sensing redundancy for the left ventricle,wherein the at least one additional electrode is operationallypositioned to the left ventricle and is used to provide pacingredundancy for the left ventricle by pacing the left ventricle atanother site if a failure or an algorithm inhibits a pace to the firstsite or to provide sensing redundancy for the heart chamber by sensingthe cardiac activation at the other site.
 8. The method of claim 7,wherein: using the first electrode includes pacing the first site of theleft ventricle using the first electrode; and using the at least oneadditional electrode includes sensing cardiac activation at the othersite of the left ventricle using the at least one additional electrodeand determining if cardiac activation at the other site occurs within anexpected conduction delay window from a time when the first site ispaced.
 9. The method of claim 7, wherein: using the first electrodeincludes sensing the cardiac activation at the first site of the leftventricle using the first electrode; and using the at least oneadditional electrode includes sensing cardiac activation at the othersite of the left ventricle using the at least one additional electrodeand determining if the cardiac activation at the other site occurswithin an expected conduction delay window from the cardiac activationat the first site.
 10. A method for using at least two electrodesimplanted in or proximate to a heart chamber, comprising: using a firstelectrode operationally positioned proximate to a first site of theheart chamber to pace the first site or to sense a cardiac activation atthe first site; sensing at the first site; and using a second electrodeoperationally positioned proximate to a second site of the heart chamberto provide pacing redundancy for the heart chamber based on the sensingat the first site.
 11. The method of claim 10, wherein the firstelectrode is operationally positioned to pace a first left ventricularsite and the second electrode is operationally positioned to pace asecond left ventricular site.
 12. A method for using at least twoelectrodes implanted in or proximate to a heart chamber, comprising:using a first electrode operationally positioned proximate to a firstsite of the heart chamber to pace the first site or to sense a cardiacactivation at the first site; verifying electrical propagation in hearttissue attributed to the pace or the sensed cardiac activation usingmultiple site cross-checking; and using a second electrode operationallypositioned proximate to a second site of the heart chamber to providepacing redundancy for the heart chamber based on the multiple-site crosschecking.
 13. The method of claim 12, wherein the first electrode isoperationally positioned to pace a first left ventricular site and thesecond electrode is operationally positioned to pace a second leftventricular site.
 14. The method of claim 13, wherein verifyingelectrical propagation includes verifying electrical propagation in theleft ventricle using multiple site cross-checking.
 15. A method forusing at least two electrodes implanted in or proximate to a heartchamber, wherein the at least two electrodes include a first electrodeand a second electrode, the method comprising: pacing a first site ofthe heart chamber using the first electrode; and verifying that thefirst site of the heart chamber is paced, wherein verifying includessensing cardiac activation at a second site of the heart chamber usingthe second electrode.
 16. The method of claim 15, wherein the first andsecond electrodes are on the same lead.
 17. The method of claim 15,wherein verifying includes determining if the cardiac activation at thesecond site occurs within an expected conduction delay window.
 18. Themethod of claim 17, further comprising pacing the second site of theheart chamber if the cardiac activation is not sensed at the second sitewithin the expected time delay window.
 19. The method of claim 15,including determining that the first site was not paced because of afailure condition if cardiac activation does not occur within anexpected conduction delay window.
 20. A method for using at least twoelectrodes implanted in or proximate to a heart chamber, wherein the atleast two electrodes include a first electrode and a second electrode,the method comprising: sensing a cardiac event at a first site of theheart chamber using the first electrode; and verifying the sensedcardiac event, wherein verifying includes sensing cardiac activation ata second site of the heart chamber using the second electrode.
 21. Themethod of claim 20, wherein verifying includes determining if cardiacactivation at the second site occurs within an expected conduction delaywindow.
 22. The method of claim 20, including determining that thesensed event did not occur because of a failure condition if cardiacactivation did not occur within an expected conduction delay window. 23.The method of claim 22, wherein the failure condition includes noise oroversensing.
 24. An implantable medical device, comprising: at least afirst electrode and a second electrode; means for using the firstelectrode operationally positioned proximate to a first site of a heartchamber to pace the first site or to sense a cardiac activation at thefirst site, wherein the second electrode is not used with the firstelectrode to pace the first site or to sense the cardiac activation atthe first site; and means for using the second electrode operationallypositioned proximate to a second site of the heart chamber to providepacing redundancy for the heart chamber by pacing the second site if afailure or an algorithm inhibits a pace to the first site or to providesensing redundancy for the heart chamber, wherein the second electrodeis used instead of the first electrode to pace the heart or to sense thecardiac activation.
 25. The device of claim 24, wherein: the means forusing the first electrode includes means for pacing the first site ofthe heart chamber using the first electrode; and the means for using thesecond electrode includes means for sensing cardiac activation at thesecond site of the heart chamber using the second electrode and meansfor determining if cardiac activation at the second site occurs withinan expected conduction delay window from a time when the first site ispaced.
 26. The device of claim 24, wherein: the means for using thefirst electrode includes means sensing the cardiac activation at thefirst site of the heart chamber using the first electrode; and the meansfor using the second electrode includes means for sensing cardiacactivation at the second site of the heart chamber using the secondelectrode and means for determining if the cardiac activation at thesecond site occurs within an expected conduction delay window from thecardiac activation at the first site.
 27. An implantable medical device,comprising: at least one implantable lead configured to position atleast a first electrode and a second electrode in or proximate to aheart chamber; a pace output circuit configured to pace a first site ofthe heart chamber using the first electrode; a sensing circuitconfigured to sense cardiac activation at a second site of the heartchamber using the second electrode; a control circuit connected to thepace output circuit and to the sensing circuit, wherein the controlcircuit is configured to verify that the first site of the heart chamberis paced to induce a conducted event by sensing the conducted event atthe second site.
 28. The device of claim 27, wherein the at least oneimplantable lead includes one lead with both the first and secondelectrodes.
 29. The device of claim 27, wherein the pace output circuitis configured to pace the second site of the heart chamber using thesecond electrode, wherein the controller is configured to pace thesecond site of the heart chamber using the second electrode if theconducted event is not sensed at the second site within an expectedconduction delay window.
 30. The device of claim 27, wherein the controlcircuit is configured to verify that the first site of the heart chamberis paced if the conducted event is sensed within an expected conductiondelay window.
 31. An implantable medical device, comprising: at leastone implantable lead configured to position at least a first electrodeand a second electrode in or proximate to a heart chamber; a firstsensing circuit configured to sense cardiac activation at a first siteof the heart chamber; a second sensing circuit configured to sensecardiac activation at a second site of the heart chamber; and a controlcircuit connected to the first and second circuits, wherein the controlcircuit is configured to verify an event sensed using the first sensingcircuit by sensing the event using the second sensing circuit.
 32. Thedevice of claim 31, wherein the control circuit is configured to verifythe event sensed using the first sensing circuit if the conducted eventis sensed using the second sensing circuit within an expected conductiondelay window.